1. Field of the Invention
The present invention relates to liquid-discharge recording heads which discharge liquid for recording, and more particularly relates to a liquid-discharge recording head which discharges liquid drops of different volumes for recording.
2. Description of the Related Art
Resolution of color inkjet printers using thermal inkjet technology is increasing every year. In particular, in recording heads for forming images, resolution of nozzle arrays for discharging ink drops has increased to 600 dpi and 1200 dpi.
In addition, the volume of ink drops discharged for forming images is reduced every year in order to reduce the granularity of half-tone areas in grayscale images and middle-tone and highlight areas in color photo images. In particular, in recording heads for discharging color ink, the volume of ink drops has been reduced from about 15 pl to 5 pl and 2 pl.
However, when rough images, such as color graphs in charts, are printed and the required resolution is relatively low, high-resolution recording heads which discharge small ink drops cannot satisfy demands for high-speed printing. This is because the amount of image output data is large due to small drop size and high resolution and a long data transmission time is necessary.
In order to solve this problem, images may be formed with relatively large ink drops and small output data size in high-speed printing. In high-quality printing, the size of the ink drops may be changed so as to make the granularity in images as low as possible. Accordingly, there is a demand to change the size of ink drops of each color using a recording head that has a nozzle group for discharging ink drops of different sizes.
In order to satisfy this demand, Japanese Patent Laid-Open No. 8-183179 (corresponding to U.S. Pat. No. 6,309,051), for example, discloses a structure for discharging ink drops of different sizes from the same nozzle. In this structure, electrothermal transducers of different sizes are arranged in an ink passage which communicates with the nozzle, and ink drops of different sizes are discharged from the nozzle by selectively causing the electrothermal transducers to generate bubbles.
In addition, U.S. Pat. No. 6,137,502 discloses an inkjet recording head in which nozzles for discharging large ink drops and nozzles for discharging small ink drops are alternately arranged in two lines.
However, in the structure according to Japanese Patent Laid-Open No. 8-183179, since the ink drops of different sizes are discharged from the same ink passage, the ink supply speed at which the ink is supplied from behind the nozzle differs depending on the size of the discharged ink drops. Therefore, in a serial recording device, it is difficult to discharge ink drops of different sizes in a single recording-head scanning period (in a single scan). Accordingly, the recording head must scan a plurality of times to selectively discharge large, medium, and small ink drops. This means that ink drops of different sizes cannot be discharged at the same frequency, and therefore it is difficult to perform drop-size modulation control when high-definition images are formed.
In addition, in the structure according to U.S. Pat. No. 6,137,502, the same number of large and small nozzles is provided. Therefore, although high-speed printing using large amounts of ink can be performed without any problem when the amount of discharge is set large, the image quality is reduced in high-quality tone printing (photo printing). In addition, although the image quality can be increased in photo printing when the amount of discharge is set small, the print speed is reduced since the number of print passes is increased.